1. Field of the Invention
The present invention relates to a solar cell and, more particularly, to a solar cell in which its electrode structure is improved, output characteristics are stably improved, and reliability is also improved.
2. Related Background Art
In recent years, strong worldwide demand has arisen for increased power supply, and active power generation to answer this demand now poses a serious problem of environmental pollution caused by the power generation.
Under these circumstances, a power generation scheme of solar cells using sunlight has received a great deal of attention as a clean power generation scheme in which problems posed by terrestrial warming caused by radioactive contamination and emission of gases having a greenhouse effect can be solved, energy sources are less localized because sunlight is radiated on the entire area of the earth, and relatively high power generation efficiency can be obtained without requiring complicated bulky equipment. Thus, this power generation scheme is expected to cope with increased future power supply demand without destroying the environment, and various studies have been made for practical applications of the scheme.
In order to establish a power generation scheme which uses solar cells, and which can satisfy power supply needs, the solar cell must have sufficiently high efficiency of photoelectric conversion and stable characteristics and must allow mass-production as the basic requirements.
For this reason, the following solar cell and manufacturing method therefor have received a great deal of attention. That is, an easily accessible source gas such as silane is used and decomposed by a glow discharge or the like to deposit a semiconductor thin film such as non-monocrystalline silicon on a relatively inexpensive substrate such as a glass substrate or a metal sheet, thereby mass-producing solar cells. These solar cells can be manufactured with a small amount of energy at low cost as compared with a monocrystalline silicon solar cell or the like.
The study of applications of non-monocrystalline silicon to photovoltaic elements such as solar cells was started with the invention of a solar cell by D. E. Carlson (U.S. Pat. No. 4,064,521) based on the success of doping by W. E. Spear and P. G. LeComber (Solid State Communications, Vol. 17, pp. 1193-1196, 1975). Although the field of solar cells has a short history of research and development, a variety of fruitful studies have been reported.
Semiconductor layers as important constituent elements of a solar cell form semiconductor junctions such as the so-called p-n and p-i-n junctions. These semiconductor junctions can be formed by sequentially stacking semiconductor layers having different conductivity types, or incorporating a dopant having a conductivity type different from that of a given semiconductor layer into said semiconductor layer in accordance with ion implantation or the like, or by diffusion of a dopant by thermal diffusion. A solar cell obtained using a thin film semiconductor such as amorphous silicon as the non-monocrystalline silicon described above has been studied. In the manufacture of the solar cell according to known techniques, a source gas containing an element serving as a dopant such as phosphine (PH.sub.3 for n-type semiconductor) or diborane (B.sub.2 H.sub.6 for p-type semiconductor) is mixed with silane or the like serving as a main source gas, and the gas mixture is decomposed by a glow discharge or the like to obtain a semiconductor film having a desired conductivity type. Such semiconductor films are sequentially stacked on a desired substrate in a p-i-n or n-i-p structure, thereby easily obtaining a semiconductor junction.
As a result of these studies, solar cells using non-monocrystalline silicon have already been used in a variety of power generation applications, e.g., compact devices such as a wristwatch/clock, a compact calculator, and a streetlight. When non-monocrystalline silicon is to be applied to a large device for power generation, many problems left unsolved (e.g., lower conversion efficiency than that of a monocrystalline or compound semiconductor solar cell, and degradation) must be solved. These problems are posed as disadvantages of the monocrystalline silicon solar cells. Numerous attempts have been made to solve these problems.
For example, these attempts include use of p-type non-monocrystalline silicon carbide having a large forbidden band width as a incident-side window layer (Y. Uchida, US-Japan Joint Seminar, Technological Applications of Tetrahedral Amorphous Solids, Palo Alto, Calif. (1982)), use of p-type silicon carbide having fine crystal grains as a window layer (Y. Uchida et al., Technical Digest of the International PVSEC-3, Tokyo, Japan (1987) A-IIa-3, pp. 171-174), and the like.
In the use of non-monocrystalline silicon carbide having a large forbidden band width as a window layer, there is attempted a method of forming a so-called graded (i.e., the forbidden band width is continuously changed) buffer layer in which an energy band step formed at the p-i interface is eliminated to prevent the efficiency of photoelectric conversion from being degraded, in a short-wavelength range, the degradation being caused by redistribution or recombination of the carriers (R. R. Arya et al., Technical Digest of the International PVSEC-3, Tokyo, Japan (1987) A-IIIa-4, pp. 457-460).
According to another attempt, phosphorus atoms (P) or boron atoms (B) are doped in an i-type layer in a very small amount of 10 ppm or less to increase the carrier mobility in the i-type layer (W. Kuwano et al., The Conference Record of the Nineteenth IEEE Photovoltaic Specialists Conference-1987, p. 599; M. Kondo et al., The Conference Record of the Nineteenth IEEE Photovoltaic Specialists Conference-1987, p. 604).
According to still another attempt, p- and n-type dopants are diffused in a semiconductor layer having another conductivity type to weaken the semiconductor junctions at the p-n, p-i, and n-i interfaces, thereby preventing degradation in efficiency of photoelectric conversion of a photovoltaic element. An example of such an attempt is disclosed in Japanese Laid-Open Patent Application No. 55-125681 (applicant: Sanyo Electric Co., Ltd.). This prior art discloses a method of forming a solar cell by arranging a partition door in a plasma reaction chamber through which a glass substrate on a conveyor passes. U.S. Pat. No. 4,266,897 (Assignee: Plasma Physics Inc.) discloses a method in which spaces for forming respective semiconductor layers are separated from each other by gas gates (i.e., a mechanism in which a gas which does not contribute to film formation is forcibly strongly flowed to serve as partition walls for the film formation gases) in an apparatus for continuously forming a solar cell on an elongated substrate, thereby preventing a dopant from entering into the wrong film formation space.
Japanese Laid-Open Patent Application No. 56-69875 (applicant: Fuji Electric Col, Ltd.) discloses a method in which a transparent conductive layer is formed between a conductive substrate and a semiconductor layer in a solar cell having the semiconductor layer on the conductive substrate to increase the adhesion strength between the semiconductor layer and the substrate, or to improve the surface smoothness of the substrate, thereby improving the characteristics of the solar cell. Japanese Laid-Open Patent Application No. 55-108780 (applicant: Sharp Corporation) discloses a method in which a decrease in reflectance at an interface caused by formation of an alloy between the semiconductor atoms and a diffused metal element constituting the lower electrode located on the surface of a semiconductor layer at a side opposite to the light-receiving surface can be prevented by forming a transparent conductive layer such as a zinc oxide layer between the lower electrode and the semiconductor layer.
According to still another attempt, tin or indium is doped in a zinc oxide layer to decrease the resistivity of the zinc oxide layer (C. X. Qiu, I. Shin, Solar Energy Materials, Vol.. 13, No. 2, pp. 75-84, 1986). An example of aluminum doping in a zinc oxide layer is also reported (Igasaki & Shimaoka, Research Report of Shizuoka University, Electronic Engineering Lab., Vol. 21, No. 1, 1986), and an example of fluorine doping is also reported (Koinuma et al., Transactions of the Society of Japanese Ceramics, Vol. 97, No. 10, pp. 1160-1163, 1989).
As a result of efforts made by a number of engineers in addition to the above attempts, the disadvantages (e.g., low efficiency of photoelectric conversion and degradation) of non-monocrystalline silicon solar cells have been improved. However, the following problems are still left unsolved.
When a solar cell is formed by forming a semiconductor layer on a conductive substrate through a zinc oxide layer, the adhesion strengths between the zinc oxide layer and the conductive substrate and between the zinc oxide layer and the semiconductor layer are not sufficient. Slight peeling may occur due to temperature shocks and vibrations in formation of the semiconductor layer and the subsequent step. This peeling poses a problem of initial characteristics, i.e., degradation in efficiency of photoelectric conversion of the solar cell.
In addition, since the resistivity of the zinc oxide layer cannot be reduced to a negligible degree, an increase in series resistance of the solar cell can be expected, thus posing a problem of initial characteristics, i.e., degradation in efficiency of photoelectric conversion of the solar cell.
This also applies not only to formation of the semiconductor layer on the conductive substrate but also to a solar cell in which a transparent conductive layer is formed on a transparent insulating substrate and a semiconductor layer is formed on the transparent conductive layer. More specifically, slight peeling occurs between the transparent conductive layer and the semiconductor layer even during the manufacture due to insufficient adhesion strength between the transparent conductive layer and the semiconductor layer, thereby posing a problem of initial characteristics, i.e., degradation in efficiency of photoelectric conversion of the solar cell.
Even if slight peeling does not occur in the manufacture of a solar cell and efficiency of photoelectric conversion in the initial manufacturing period of the solar cell is rather high, slight peeling may occur between the conductive substrate and the transparent conductive layer and between the transparent conductive layer and the semiconductor layer in practical application states under various climatic and installation conditions, thereby degrading reliability due to gradual degradation in efficiency of photoelectric conversion of the solar cell.